WATER FEBRUARY 2014
72
Technical Papers
irrigation than Melbourne and
Hobart) is more pronounced than in
the theoretical case. With the modi ed
irrigation rate, winter-growing crops
that theoretically needed little to no
irrigation now receive irrigation because
the gardener is assumed to be watering
indiscriminately across the entire planted
area based on the crop with the highest
water requirement.
For our hypothetical food garden (see
Table 2), the modi ed irrigation rates can
be applied to determine overall water
consumption each month. The resultant
irrigation rates are shown in Figure 1.
The most dramatic feature of this graph
is that in all cities there is a marked drop
in water use from May to August. This
occurs because the deciduous trees
(peach, pear, plum and almond) do not
require water during these months and
the remaining crops have low water
requirements at this time of year. It is
also interesting to note that in the other
months, the modi ed irrigation rate stays
relatively constant at about 140--160 L/
day in Adelaide and Perth, and about
80--100 L/day in Melbourne and Hobart.
Finally, in Table 6 we show the overall
water use and cost of water in our
hypothetical food garden. Baseline
water consumption is based on 2011--
2012 per-capita water consumption
(ABS, 2013a) and an assumed 2.6 people
per household (national average). In
this analysis Adelaide has by far the
highest irrigation water cost due to
a combination of high price and high
crop water demand. It is interesting
that Perth, despite having the highest
water requirement of all four cities, has
an overall irrigation cost comparable
to that of Melbourne, due to its low
water prices. Hobart has only a single,
comparatively low water price for its
mains water, and coupled with its low
crop water use, the cost of growing food
in this city is by far the lowest of the four.
DISCUSSION
It should be noted that several factors
may point to this analysis being overly
pessimistic with respect to water use in
UA. First, as a result of applying extra
water (as in our modi ed irrigation
rate), the yield for a number of crops
may be higher than the assumed
average yield. Moreover, in an intensive
ecological polyculture system such as
"permaculture" (Mollison and Holmgren,
1978), it may be possible for deep-rooted
plants to capture and utilise surplus
irrigation water after it has drained past
the root zone of adjacent shallow-rooted
crops, leading to conservation of water
use ef ciency by the whole system,
despite apparent over-irrigation of
individual components.
Notwithstanding these possibilities,
we contend that the modelling approach
presented in this study (using yields
based on commercial production and
irrigation based on standard crop water
requirements) remains a reasonable
rst attempt to quantify the likely water
footprint of urban food production,
given the paucity of data on this subject.
Table 5. Modi ed irrigation footprint of crops grown in different cities.
Modi ed irrigation water requirement (L/kg)
Crop
Adelaide Melbourne
Hobart
Perth
Almonds
8073
5172
5007
8744
Green beans
998
563
562
844
Broccoli (winter)
654
338
537
688
Broccoli (summer)
981
662
647
887
Carrot (winter)
205
110
142
166
Carrot (summer)
326
216
142
355
Cauli ower
325
176
292
342
Garlic
590
398
392
645
Lettuce (winter)
285
157
165
272
Lettuce (summer)
262
162
147
292
Oranges
751
469
459
806
Peach
1823
1168
1131
1974
Pear
562
360
349
609
Plum
1662
1065
1031
1800
Pumpkin
374
248
270
410
Spinach (winter)
384
219
278
763
Spinach (summer)
567
394
489
262
Tomato
141
101
98
154
0
20
40
60
80
100
120
140
160
180
200
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Modified irrigation rate (L/day)
Adelaide Melbourne Hobart Perth
Figure 1. Modi ed irrigation rate (per capita) for a planted area of hypothetical
food garden (25m2 of fruit and nut trees and approximately 10m2 of seasonal
vegetables).
AGRICULTURE & FOOD